Global energy consumption is projected to significantly increase by mid-century, and this increased need may be partially met through use of renewable energy sources. Due to the intermittent nature of some of these renewable energy sources, such as wind and solar, it is desirable to incorporate compatible large-scale energy storage devices into the energy grid. Use of such grid storage is also being driven by the evolving features of the electrical grid, such as green grid technology, smart grid technology, and a distributed structure of the grid, as well as by other technological developments including electric vehicles.
In conventional flow batteries, electrolyte (e.g., catholyte and anolyte) that includes one or more dissolved electroactive species oftentimes flows through an electrochemical cell that reversibly converts chemical energy to electricity. The electroactive components are dissolved in a solvent rather than being in a solid state in such flow batteries. The electrolyte is stored external to the cell (e.g., in tanks), and can be pumped through the cell or fed into the cell via gravity. Thus, spent electrolyte in the cell is recovered for re-energization and replaced with electrolyte from the external tanks Conventionally, charge is stored and drawn from the electrolyte solution. While flow batteries may be charged and discharged without degradation of performance, conventional flow batteries commonly have low energy densities and include costly materials.
There are some recent attempts to combine the infrastructure of lithium-ion batteries with the advantages of redox flow batteries (RFBs). In one example, intercalation materials and conductive additives were made into suspensions that circulate between the electrochemical cell and external storage tanks. However, the parasitic energy losses associated with pumping the highly viscous materials and the inherent deficiencies of the intercalation materials makes this system impractical for large-scale energy storage. Previous research has also reported lithium-ion RFB systems where an aqueous iron-based cathode was separated from a metallic lithium anode by a solid lithium-ion conductor, and it was pumped to flow through the cathode in a loop. Both of these systems have decreasing voltage with charge cycling, low capacity, and limited stability of the solid lithium-ion conductor. There is also a report of a battery that is a hybrid of these two systems (intercalation cathode and lithium metal anode), but it has very low loading of the cathode material and therefore very low energy density. In addition, all of the non-aqueous studies reported have energy densities more than an order of magnitude lower than state-of-the-art aqueous RFB chemistries.